Heat exchanger



FIG. 1 is a perspective view of a heat exchanger, showing our design;

FIG. 2 is a side view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a top view thereof; and,

FIG. 5 is a bottom view thereof.

The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design. 

We claim the ornamental design for a heat exchanger, as shown and described. 